Techniques have been developed which allow the investigator to alter at will the fatty acid composition of membrane lipids in animal cells grown in culture. These techniques have been applied on a small scale to cell lines that do not require serum, and more recently to cell lines that do. We intend to develop these techniques further so that we will have at hand sufficiently large amounts of cells to prepare surface membranes for physical studies, and to prepare enveloped viruses in sufficiently large quantities for physical studies. The physical studies will include esr with TEMPO and hydrocarbon probes, and also x- ray diffraction. The membrane phase transitions will be determined in viral (NDV) preparations as a prelude to studies to determine whether the infectious particle enters the cell by fusion or by phagocytosis. The ability to vary the fatty acids independently in cells and viruses will allow us to distinguish between a purely cell mediated means of entry (phagocytosis) and a cooperative cell-virus mediated means of entry (fusion).